Apoquel contains one active ingredient: oclacitinib maleate, a synthetic compound designed to block the specific immune signals that make dogs itch. It’s classified as a JAK inhibitor, meaning it interrupts the communication pathway between immune cells that triggers allergic itching and inflammation. Beyond that active ingredient, each tablet contains a handful of inactive fillers, binders, and flavoring agents that hold the pill together and make it palatable.
The Active Ingredient: Oclacitinib Maleate
Oclacitinib maleate is the only drug compound in Apoquel. It belongs to a class of medications called Janus kinase (JAK) inhibitors, which work by blocking enzymes inside cells that relay signals from the immune system. Specifically, oclacitinib is most potent against an enzyme called JAK1, with roughly 10 times more selectivity for JAK1 than for a related enzyme called JAK3. This selectivity matters because JAK1 is the enzyme responsible for transmitting signals from the cytokines that drive allergic itch in dogs.
The cytokines oclacitinib targets include IL-31, which is the primary itch-signaling molecule in canine allergic dermatitis, along with IL-2, IL-4, IL-6, and IL-13, all of which play roles in inflammation. In laboratory studies using immune cells from allergic dogs stimulated with house dust mite allergen, oclacitinib was able to cut IL-31 secretion in half at low concentrations. At the same time, the drug’s selectivity for JAK1 means it largely avoids suppressing other immune signals that rely on different enzymes, like those involved in red blood cell production.
Inactive Ingredients in the Tablets
The non-drug components in Apoquel vary slightly depending on whether your dog takes the standard film-coated tablet or the newer chewable version. The chewable tablets, which are more commonly prescribed because dogs accept them more easily, contain:
- Pork liver powder and brewer’s yeast for flavoring
- Crospovidone and sodium starch glycolate as disintegrants that help the tablet break apart in the stomach
- Magnesium stearate and glycerol monostearate as lubricants used during manufacturing
- Macrogol 3350, glycerol, and xanthan gum as binding and texture agents
- Sodium chloride (table salt) for palatability
- Colloidal silica as an anti-caking agent
These are standard pharmaceutical excipients. The pork liver powder is worth noting if your dog has a known pork sensitivity, though this is uncommon.
How the Dosing Works
Apoquel is dosed by body weight at 0.4 to 0.6 mg of oclacitinib per kilogram (0.18 to 0.27 mg per pound). The tablets come in several fixed sizes, and your vet selects the one that lands your dog’s dose within that range. For the first 14 days, the tablet is given twice daily as a loading phase. After that, it drops to once daily for ongoing maintenance.
This twice-to-once schedule isn’t arbitrary. At once-daily dosing, the drug’s blood levels stay high enough to keep blocking JAK1-dependent itch signals but drop low enough between doses to avoid suppressing non-JAK1 immune pathways more than necessary. Many owners report visible itch relief within the first few hours of the initial dose, which is considerably faster than older allergy medications like cyclosporine.
Side Effects Seen in Clinical Trials
In FDA clinical trials involving over 400 dogs, the most commonly reported side effects were mild and digestive. During the first two weeks of treatment, diarrhea occurred in about 2 to 5% of dogs on Apoquel, vomiting in 2 to 4%, loss of appetite in 1 to 3%, and lethargy in about 2%. These rates were only slightly higher than what was seen in dogs receiving a placebo, suggesting many of these symptoms may not have been caused by the drug at all.
Longer-term data paints a broader picture. In a group of 283 dogs followed over extended treatment periods, the most frequently noted issues were skin infections (12%), new skin lumps (12%), ear infections (10%), vomiting (9.2%), and diarrhea (6%). Skin infections and ear infections are also extremely common in allergic dogs regardless of treatment, so teasing apart what the drug caused versus what the underlying allergy caused is difficult. Still, the pattern points to something important about how Apoquel works: by dialing down part of the immune response to stop itching, it can also reduce the body’s ability to fight off certain infections.
Dogs That Shouldn’t Take Apoquel
Apoquel is not approved for dogs under 12 months of age. Young dogs have developing immune systems, and suppressing JAK1 signaling during that window carries risks that haven’t been adequately studied. It’s also contraindicated in dogs with serious infections, since the drug’s immune-modulating effects can make existing infections harder to clear. Dogs with pre-existing cancers are another concern, as Apoquel may allow tumors to progress more readily when immune surveillance is dampened.
Breeding, pregnant, and lactating dogs should not take it either. And because oclacitinib affects the immune system, it hasn’t been evaluated for safety when combined with other immunosuppressive drugs like cyclosporine or corticosteroids at full doses.
What Long-Term Use Looks Like
Many dogs with environmental allergies stay on Apoquel for months or years, either continuously or seasonally. For dogs on long-term maintenance, periodic bloodwork is recommended to monitor for changes in immune cell counts or organ function. The specific schedule depends on your vet’s judgment and your dog’s health history, but a complete blood panel once or twice a year is a common approach.
The 3.9% rate of histiocytomas (a type of benign skin tumor) seen in longer-term trials, along with the 2.1% rate of lipomas, has fueled ongoing discussion about whether JAK inhibition raises cancer risk over time. These tumor types are common in dogs generally, which makes it hard to draw firm conclusions from the available data. A review published in the Journal of the American Veterinary Medical Association, looking back on a decade of clinical use, noted that oclacitinib’s selectivity for JAK1 helps limit off-target immune suppression, but acknowledged that vigilance around new lumps remains sensible for any dog on the medication long-term.